1. Field of the Invention
This invention generally relates to Global Positioning System (GPS) asset localization and, more particularly, to a method of minimizing the number of GPS satellite signals needed or to improve accuracy in GPS-based asset localization.
2. Background Description
The tracking and location of assets such as railcars, shipping or cargo container, trucks, truck trailers, automobiles, etc. can be highly advantageous in commerce. Precise tracking of such vehicles and objects can facilitate their being allocated and positioned in an efficient manner, and can provide for immediate, accurate localization of lost, delayed or damaged assets. The space-based global positioning system (GPS) implemented by the United States Department of Defense constitutes a convenient instrumentality for determining geographical position in real time.
The GPS is a multiple satellite-based radio positioning system in which each satellite transmits data that allows precise measurement of the distance from selected ones of the GPS satellites to the antenna of a user's receiver so as to enable the user to compute position, velocity and time parameters through known triangulation techniques. The signals provided by the GPS can be received both globally and continuously.
The GPS comprises three major segments known as space, control and user segments. The space segment consists of twenty-one operational satellites and three spare satellites. The satellites are positioned in a constellation such that typically seven satellites, but a minimum of four, are observable by a user anywhere on or near the earth's surface. Each satellite transmits L-band signals on two frequencies known as L1 (1575.42 MHZ) and L2 (1227.6 MHZ), using spread spectrum techniques that employ two types of spreading functions.
C/A (or coarse/acquisition code) and P (or precise) pseudo random noise (PRN) codes are transmitted on frequency L1, and P code only is transmitted on frequency L2. Both P and C/A codes contain data that enable a receiver to determine the range between a satellite and the user.
Superimposed on both the P and C/A codes is a navigation (NAV) message. A NAV message contains the GPS signal transmission time, a handover word used in connection with the transition from C/A to P code tracking, ephemeris (position) data for the given satellite, and almanac data for all of the satellites in the constellation, including information regarding satellite health, coefficients for the ionospheric delay model for C/A code users, and coefficients used to calculate universal coordinated time (UCT).
The control segment comprises a master control station (MCS) and a number of monitor stations. The monitor stations passively track all GPS satellites in view, collecting ranging data and satellite clock data from each satellite. This information is passed on to the MCS where the satellite's future ephemeris and clock drift are predicted. Updated ephemeris and clock data are uploaded to each satellite for retransmission in each satellite's navigation message. The purpose of the control segment is to ensure that the information transmitted from the satellites is as accurate as possible.
The GPS is useful in a wide variety of applications, including space, air, sea and land vehicle navigation, precise positioning, time transfer, altitude referencing and surveying. A typical GPS receiver comprises several subsystems, including an antenna assembly, an RF (radio frequency) assembly, and a GPS processor assembly. The antenna assembly receives the L-band GPS signal and amplifies it prior to insertion into the RF assembly. The RF assembly mixes the L-band GPS signal down to a convenient IF (intermediate frequency) signal. This signal is then fed to the GPS processor assembly in which the PRN code modulating the L-band signal is tracked through code-correlation at the receiver. This provides the processing gain needed to achieve a signal-to-noise ratio (SNR) sufficient for demodulating the navigation data and signal-transmission time stamp. The Doppler shift of the received L-band signal is also measured through a carrier tracking loop. The code correlation and carrier tracking function can be performed using either analog or digital signal processing.
A GPS receiver can determine its location by processing signals received from several of the GPS satellites so as to determine the satellite positions and range from the receiver to each of the satellites. With perfect knowledge of the range to just three satellites, exact receiver position can be determined as the intersection of the three spheres induced by the known satellite positions and the derived receiver ranges. The range to a particular satellite is calculated as the propagation time multiplied by the speed of light. The propagation time is found by recognizing a predetermined transmitted marker in the received signal and noting its reception time, reading the transmission time associated with the marker from the NAV data stream, and calculating propagation time as the difference between the reception and transmission times. The reception time is found by performing a cross-correlation of the received signal with a local replica of the known satellite Gold code, and noting the reception time of a chosen correlation peak. The satellite signal structures use Code Division Multiple Access (CDMA) coding so that the above cross correlation is part of the standard GPS receiver processing.
One type of known GPS receiver is described in U.S. Pat. No. 4,114,155, wherein the position of the receiver responsive to C/A signals derived from multiple, orbiting satellites is determined to an accuracy better than 100 meters. Each of the C/A signals has the same transmitted carrier frequency and a different, predetermined Gold code sequence. C/A signals transmitted to the receiver are separately detected by cross-correlating received Gold code sequences with plural locally derived Gold code sequences. Four of the detected C/A signals are combined to compute receiver position. To determine receiver position to an accuracy typically better than 100 meters, the cross correlation is repeated while the relative phase of the locally derived Gold code sequences is varied over the interval of one chip (i.e., pulse) of each sequence. This provides for sub-chip accuracy in the signal timing.
In operation, a typical GPS receiver performs the following for each of at least four satellite signals:
1) acquires the GPS signal, (detects the signal and learns its code, code-time offset, and Doppler shift), PA1 2) synchronizes with the NAV data stream and reads the satellite transmission time-stamp, clock-correction, ionospheric-delay and ephemeris data, PA1 3) calculates the satellite position from the ephemeris data, PA1 4) reads its own receiver clock to determine the receiver time associated with the reception of the time-stamp epoch, and PA1 5) estimates the signal travel time by subtracting the time-stamp value from the associated receiver time. This time difference is multiplied by the speed of light to obtain an estimated range to the satellite.
Any error in satellite and receiver clock synchronization leads to proportional range errors. Because the same clock is used in receiving all satellite signals, there is only one unknown receiver clock "bias" or error to be found. By receiving and processing a fourth satellite signal, four measurement equations are generated and these are used to jointly solve for the clock bias and the three receiver position coordinates. Error induced by receiver noise is reduced by (effectively) averaging over several range calculations.
A preferred application of the present invention is the locating and tracking of assets such as rail cars, shipping or cargo containers, trucks, truck trailers, and the like, using the GPS. In such application, the GPS receiver is usually battery powered since an independent source of power is generally not available. It is advantageous to increase the operating life of the battery by reducing the energy consumed by the GPS receiver.
In one system of the invention, a central facility or station must track multiple railcars. Each tracked railcar carries a GPS receiver that processes data from several of the visible GPS satellites; however, an accurate position determination is not made at the receiver. Instead, only partial processing is performed at the receiver and intermediate results are transmitted from the railcar to the central station. These intermediate results do not require decoding of navigational or other data from the GPS signals. This system thus allows the GPS receiver and signal processor to be powered only long enough to acquire the GPS satellite signals. With this system, a dominant energy consumer is the GPS signal acquisition process, and the GPS receiver energy used at each tracked railcar will be reduced if the number of GPS satellite signals needed is reduced.
The above referenced application Ser. No. 08/456,229 discloses two methods of centralized object tracking that do not require demodulation of the received GPS signals NAV data. These methods thus allow the GPS receiver and signal processor to be powered only long enough to acquire the GPS satellite signals. In the first method, five GPS satellite signals must be received. The code or bit phases are measured for each signal and sent to the central station along with their associated satellite identification numbers. From these measurements, the asset location is determined at the central station. In the alternative second method, only four GPS satellite signals must be received. The reception time and the code or bit phases are measured for each signal and sent to the central station along with their associated satellite numbers. From these measurements, the asset location is determined at the central station.
It is advantageous to further reduce the number of GPS satellite signals that must be received by a tracked asset such as a railcar. Railcars are frequently in positions where trees, buildings, and the like block reception of some GPS signals so that fewer than the normally required four or more GPS satellite signals are receivable. In such cases, it would be advantageous for the central station to be able to determine railcar location from the limited number of GPS signal measurements. Furthermore, in the above referenced centralized tracking system, the acquisition consumption of energy (and other costs) associated with transmitting the measurements to the central station will be reduced at the tracked railcar if the number of GPS measurements needed is reduced.